Many attempts have been made to derive useful energy from the wind. In general, design has progressed to the point that, on a per-energy-unit cost basis, wind turbine energy costs approach those of conventional power sources. Because of the size of worldwide power needs, even small incremental changes in wind turbine efficiency can provide substantial benefits.
One factor affecting the operation and economics of wind turbines is variability. The variable nature of wind has, in the past, required devices having sufficient structural strength to withstand the peak loads such as surges in the amount of torque developed (e.g., from wind gusts and the like). Thus, many previous devices required large and strong structures to accommodate, e.g., torque surges, even though, for the majority of the time, such large and strong structures are not needed (i.e. during non-gust conditions). Variability can also contribute to undesirable movement of blades, potentially in a direction towards the tower or other structures, which, if not properly accounted for, can result in a tower strike or other possibly damaging event(s). Furthermore, torque surges result in undesirable power surges. Accordingly, it would be useful to provide a wind turbine which can effectively control torque surges and, thus, reduce the occurrence of power spikes and reduce the need for heavy and strong structures.
Some previous wind turbine designs provided for “teetering” of the blade structure (i.e., pivoting of the blades, as a unit, such that all blades pivot together) with respect to the axis of rotation, e.g., to accommodate non-uniform wind inflow conditions such as wind shear. However, some approaches for limiting the amount of teeter motion tend to provide an undesirable amount of stress on parts, sometimes leading to fatigue and/or failure of components. Further, a teetering wind turbine can have instability in low velocity wind conditions or during a high wind velocity restart.
Some wind turbine configurations permit blades to move with a “flap” motion about a flap axis which is substantially perpendicular to the rotation axis (i.e., through a range of flap angles). As used herein, “flap angle” of a blade refers to the angle measurement between the longitudinal (or pitch) axis of a blade and a plane perpendicular to the rotor shaft axis (the rotational axis) and containing the flap hinge axis associated with the particular blade. U.S. Pat. No. 5,584,655 (incorporated herein in its entirety) includes a description of providing hydraulic cylinders and the like, controllable to permit or drive flap motion e.g., in such a way as to help to reduce cyclic flapping motion of the blades. In general, blades should be actively pitched in the direction of feather (i.e., to reduce aero-dynamic lift) in high wind conditions. U.S. Pat. No. 5,584,655 further describes pitch motion being provided in response to flap motion of the blade 104b. Although a number of factors are described as affecting the coupling ratio between flap motion and pitch motion, U.S. Pat. No. 5,584,655 discloses a system in which the ratio is determined by the design. The pitch flap coupling ratio varies as a function of both pitch actuator position and flap angle. For turbine operation in the critical region around cut-out wind speed, i.e., the maximum operating wind speed for power production, the invention described in U.S. Pat. No. 5,584,655 results in relatively modest variations of pitch-flap coupling ratio over the normal range of flapping motion. For example, from +4 degrees flap angle to −4 degrees flap angle, the pitch-flap coupling ratio increases from 0.7 to 0.8. The increase in pitch-flap coupling ratio, from 0.7 to 0.8, corresponds to about 14 percent. On a downwind turbine, limiting flap motion amplitude in high wind speed conditions, i.e., near cut-out wind speed, is particularly beneficial because in doing so, clearance conditions between the blade tip and the tower are substantially improved. In steady, cut-out wind speed conditions, the mean flap angle is typically around 4 degrees. A sudden decrease in wind speed will cause a corresponding rapid reduction in flap angle, i.e., the blade flaps (moves) in the direction of the tower. By inducing a pitch change in response to the flap motion, the severity of the flap angle reduction will be diminished, blade deflections will be reduced and tower clearance will be more favorable compared to identical conditions but absent such pitch response to flap motion. Although this approach is believed to be useful, e.g., for mitigating extreme excursions in rotor thrust, rotor torque and blade flap moment, it is believed that further improvements in control of pitch and flap angle are possible, including increasing the coupling ratio and/or the rate of change of the coupling ratio (as a function of flap angle), especially in low flap angle conditions (such as between about +4 degrees and −4 degrees).